There are many sources of dilute SO.sub.2 containing gases, such as, for example, tail gases from contact sulfuric acid plants, combustion gases from high sulfur fuel, and the like. These gases if directly discharged into the atmosphere contribute to air pollution and the SO.sub.2 content is wasted. It has been proposed in the past to recover a large proportion of the SO.sub.2 in a more concentrated form. This has been described, for example as in U.S. Pat. No. of Schroeder 1,901,416, in which SO.sub.2 under pressure of, for example, up to 20 atmospheres is absorbed in water. The SO.sub.2 is then stripped from the solution by means of air at a lower pressure, substantially atmospheric pressure. The gas stream with the desorbed SO.sub.2 can then be used for many purposes for which the waste gases with their very low SO.sub.2 content are unsuitable.
The Schroeder process does effect a concentration of SO.sub.2, but at the cost of compression of the dilute gas, the mass of which per unit of SO.sub.2 is relatively high, the process requiring, therefore, a large amount of power. The process of Schroeder, therefore, although perfectly operative, incurs a high power cost, which has interfered with economic operation. Another process is described in the Roberts et al U.S. Pat. No. 3,511,027, May 12, 1970. In this process the solution of SO.sub.2 in water is desorbed by a series of flash chambers at descreasing pressures with the flash vapors compressed, cooled to condense out water and finally at substantially ambient pressure constituting a product gas. It will be noted that in the Roberts et al process multiple flashing is used but no fixed stripping gas.